I think the answer you are looking for is what is called a "German Giant" morph. It is the same species as the other lizard we refer to as "bearded dragon" in the pet trade; Pogonia vitticeps. If I remember correctly the first captive beardies were produced in Germany from animals smuggled out of Australia (Australia banned export of their native wildlife many years ago). This morph was bred more for size, but I don't know if it was larger than the wild ones. I had a rescue GG years ago. He was much larger than most other beardies I see for sale these days.
Just thought I should mention, without any actual documentation stating what morph your dragon is, and no history provided it will be very difficult to actually name the morph.
Everybody with a slightly larger than average bearded dragon thinks they have a German Giant, when in fact the majority of people do not. German Giants have been crossed with so many other morphs,and usually just by pet owners rather than breeders, meaning no real records have been recorded. Coming across a true German Giant is not so common.
Either way, you have a nice healthy looking dragon on your hands